The invention relates generally to circuits for controlling current through a solenoid, motor or other load. The invention relates more specifically to such a circuit which precisely controls the average load current and is temperature compensated.
The following patent applications filed herewith have a common Detailed Description:
U.S. Ser. No. 07/782211 filed on Oct. 24, 1991 by D. J. Ashley and M. K. Demoor, entitled "Temperature Compensated Over Current And Under Current Detector" PA0 U.S. Ser. No. 07/782833 filed on Oct. 24, 1991 by D. J. Ashley, entitled "Temperature Monitoring Pilot Transistor"
Solenoids and motors are used for various purposes, and may require carefully controlled load current at one or more current levels. For example, some solenoids are used to drive print hammers in impact printers, and require two controlled levels of drive current, an initial "activation" current and a subsequent hold current. The initial activation current is relatively large to overcome the inertia and static friction of the moving parts coupled to the solenoid, and the subsequent hold current is relatively low to limit contact force or holding force of these moving parts.
U.S. Pat. No. 4,764,840 discloses a control circuit for a solenoid which circuit provides two levels of load current. A resistor is located in series with the solenoid, and the voltage across the resistor is supplied to the positive input of one comparator and the negative input of another comparator. The other inputs to the comparators are provided by a three resistor voltage divider which divides a reference voltage. Thus, the two comparators provide a window to control the voltage which drives the load current. While the reference voltage which drives the voltage divider is fixed, a one-shot injects extra current into the voltage divider during the activation current phase to raise the window for controlling the activation current.
In precision applications, it is vital to carefully control the drive current at each level particularly in view of temperature effects. Typically, there is a load transistor which conducts the load current, and the series on-resistance of the load transistor increases as the load transistor conducts because the conducted current heats the load transistor. Consequently, the changing on-resistance of the load transistor will effect the load current. There are also other factors which affect the load current. As a result, some form of feedback has been utilized to compensate for such changes in the series on-resistance of the load transistor, variations in supply voltages and the other factors. For example, a small resistor has been placed in series with the coil, and the voltage across the resistor used to measure the drive current. This measurement in turn is used to control the drive voltage. This technique has the disadvantages of power dissipation and imprecision due to the variation in resistance of the series resistor with temperature. A more recent (prior art) technique utilizes a "drain pilot" transistor which is a scaled version of the load transistor. For example, the load transistor is made of thousands of identical transistors in parallel and the pilot transistor has the same structure as the individual transistors of the load transistor and the size of hundreds of the individual transistors in parallel. The drain pilot transistor and the load transistor are both integrated into the same semiconductor "chip" and are located adjacent to each other, but the drain pilot transistor does not conduct any of the load current. Nevertheless, as the load transistor heats-up due to the load current, the pilot transistor also heats-up and the on-resistance of each changes proportionally. A constant current source feeds the pilot transistor and therefore, develops a voltage which is proportional to the ideal load current. The voltage across the pilot transistor is then compared to a voltage sensed across the load transistor. If the sensed voltage is greater than the reference voltage then the load is disconnected from the power supply for a predetermined period. During this period, the load current will decay according to an RL time constant of the load circuit such that the sensed voltage decays below the reference voltage. Then, the power supply is re-applied to the load to cause the load current to rise, and the cycle is repeated. Thus, the load current is regulated. While such control is accurate enough for many applications, the average load current can vary despite the accuracy of the reference voltage. This is because the amount of decay of the load current when the load is disconnected from the power supply depends on the resistance of the series load circuit and this resistance can neither be designed with precision nor kept constant with changes in temperature.
A general object of the present invention is to provide a circuit for accurately regulating an average load current, which circuit is temperature compensated.
A more specific object of the present invention is to provide a circuit of the foregoing type which can accurately control the average load current at different levels and/or directions of load current.
A more specific object of the present invention is to provide a circuit of the foregoing type which can accurately control the load current to an inductive load despite a variety of affects on the load circuit.